Flood sedimentation.
Drs. Nanette C. Kingma
1
N.C.KINGMA
2/17/2003
Sediment transport by floods.
Complex:
Inability to make a good / satisfactory
measurement , of rates of sediment
transport, during extreme floods.
To make estimates is questionable.
But the trend is clear!
2
N.C.KINGMA
2/17/2003
Sediment transport by floods.
„ Transport rate of bedload is proportional to
the flow with an exponent on the order 3 - 5
during extreme floods dramatic
increases in bedload.
„ The increase in suspended load may still
be greater; measurements showed a 1000fold increase during floods.
„ The greater the competence the larger the
size of gravel and boulders entrained
and transported.
3
N.C.KINGMA
2/17/2003
Particle size range.
„ Boulders
> 256 mm;
2 - 256 mm;
1/16 - 2mm;
1/156 - 1/16 mm;
< 1/156 mm
„ Gravel
„ Sand
„ Silt
„ Clay
4
N.C.KINGMA
2/17/2003
Specific stream power.
„ The energy available per unit area of the
channel bed (w) can be related to the
stream power(Ω) to give the specific
stream power(ω)
ω = Ω/ w = τ0 * v where
τ0 is channel bed shear stress
V is mean velocity
w is channel width;
5
N.C.KINGMA
2/17/2003
Critical stream power.
„ Critical stream power is the state in which
in a specific channel reach the power
available is exactly sufficient to transport
the mean available sediment load.
„ If the available power is greater than
required to transport this mean sediment
load then erosion and down cutting will
occur.
6
N.C.KINGMA
2/17/2003
Competence of different flow
velocities.
„ Hjulstrom curves
show the competence
of different flow
velocities.
„ Competence being the
maximum particle size
that can be entrained
at a specific flow
velocity.
7
N.C.KINGMA
2/17/2003
Stream competence.
Competence:
is indicated by the
largest size of
particle that a
stream can carry in
traction as bedload.
V, Q, slope.
8
N.C.KINGMA
2/17/2003
Stream capacity.
Capacity:
is the maximum
amount of debris
of a given size
that a stream can
carry in traction
as bedload.
9
N.C.KINGMA
2/17/2003
V, slope, calibre of
load. (size
distribution of all
particles)
Relation: calibre of the load capacity.
„ Suspended sediments can increase the
fluid density and therefore capacity;
„ Whereas the introduction of large particles
into a flow can decrease the total capacity.
1
0
N.C.KINGMA
2/17/2003
Actual quantities of bedlaod.
„ Actual quantities of bedload transported
may be less than the theoretical maximum,
when erosion rates are insufficient.
„ Actual load depends on;
geologic factors;
† climatological /hydrological factors;e.g.
intensity and duration of rainfall.
†
1
1
N.C.KINGMA
2/17/2003
Actual quantities of bedlaod.
„ Quantities of suspended load transport
depend on:
†
1
2
the relative availability of grains that are
sufficiently fine to be lifted above the bed by
the flows turbulence.
N.C.KINGMA
2/17/2003
Note
„ Evaluating flood hydraulics by the largest
transported particle is FRAUGHT with
PROBLEMS due to:
processes of selective entrainment from a bed
of mixed sizes;
† size restrictions of available clasts.
†
1
3
N.C.KINGMA
2/17/2003